The invention relates generally to the field of optical communications. Specifically, the invention relates to wavelength monitoring of multiple-wavelength optical signals for optical communication systems.
Optical fiber communication systems are now widely deployed for high bandwidth telecommunication and data communication systems. Recently new communications services such as the Internet, high-speed data links, video services, and wireless services have resulted in a dramatic increase in the need for bandwidth. Data traffic is increasing at a rate of 80% per year and voice traffic is increasing at a rate of 10% per year.
Modern optical fiber communication systems have high bandwidth and low transmission loss. The bandwidth of an optical fiber determines how much information can be transmitted without losing data due to degradation in the optical signal. One way of increasing bandwidth in optical fiber communications system is to increase the number of wavelengths of light propagating in the optical fiber. Wavelength division multiplexing (WDM) is an optical technology that propagates many separate signals having different carrier wavelengths simultaneously through a single optical fiber, thus effectively increasing the aggregate bandwidth per fiber to the sum of the bit rates of each wavelength.
Dense Wavelength Division Multiplexing (DWDM) is a technology that implements WDM technology with a large number of wavelengths. DWDM is typically used to describe WDM technology that propagates more than 40 wavelengths in a single optical fiber. Bandwidths greater than 1 terabits/sec have been demonstrated in DWDM based communication systems.
Each wavelength bandwidth in a WDM system occupies a certain channel spacing in the communication system. As the number of wavelengths increases, the channel width and channel spacing decreases. The more closely spaced the carrier wavelengths, the more channels that can be propagated simultaneously and the higher the aggregate bandwidth. As the spacing between the wavelengths becomes smaller, the probability of cross talk between channels increases. This cross talk is undesirable because data from one channel interferes with data from another channel, thereby causing erroneous data to be propagated in the communication system and ultimately corrupting the data at the receiver.
In order to maximize the number of available channels in a WDM communication system, each laser source must generate an optical beam having a relatively stable wavelength. The lasers used for WDM transmitters generally emit light at a stable wavelength and the wavelength can be precisely controlled. However, many laser sources experience wavelength drift over time caused by temperature, aging, and modal instability. Wavelength drift can cause cross talk and result in a loss of data in WDM communication systems and, therefore, must be monitored and compensated.
Wavelength monitoring of the channel signals can detect drifts in the predefined channel wavelengths and can verify that channel signals added to the multiple wavelength light are within acceptable wavelength deviations of the predefined channel wavelengths. One prior art method of wavelength monitoring is to use multiple wavelength meters to-monitor the wavelength of channel signals. These meters are physically large and, consequently, are difficult to integrate into most WDM optical communication systems.
Another prior art method of wavelength monitoring uses optical spectrum analyzers. Many prior art optical spectrum analyzers use rotating gratings and/or optical filters. These systems are also physically large and, consequently, are difficult to integrate into most WDM optical communication systems. Other prior art optical spectrum analyzers use InGaAs/lnP photodetector arrays, but these spectrum analyzers are very expensive and, consequently, are not practical in integrate into most WDM optical communication systems.
U.S. Pat. No. 5,850,292 describes a prior art wavelength monitor for optical signals. In this prior-art monitor, an incident multi-wavelength signal is cascaded along a multi-point travel path in a zigzag pattern though an optically transmissive member. The multi-wavelength signal is incident on a series of wavelength discriminators arranged on the optically transmissive member. The wavelength discriminators can be implemented with a filter, such as a Fabry Perot filter. Each of the wavelength discriminators is selectively transmissive to light signals within a predetermined wavelength range containing a predetermined one of the channel signals. The wavelength discriminators are reflective to the remaining components channel signals within the multi-wavelength light signal. Each selectively transmitted channel signal is then intercepted by a detector, which produces an output current that is mapped to corresponding signal wavelengths.
The present invention relates to wavelength monitoring of multiple-wavelength optical signals propagating in an optical communication system. It is an object of the invention to provide a multi-channel wavelength monitor that is relatively inexpensive to manufacture and that can be easily integrated into optical fiber communication systems, such as DWDM communication systems. It is another object of the invention to provide a multi-channel wavelength monitor that uses a single optical filter. It is another object of the invention to provide simultaneous or parallel multi-channel wavelength monitoring. It is yet another object of the invention to provide a multi-channel wavelength that use differential detection methods to produce high wavelength resolution.
The present invention has numerous advantages over prior art multi-channel wavelength monitors. One advantage of the multi-channel wavelength monitor of the present invention is that it substantially simultaneously measures the intensity of each channel in the multi-wavelength optical beam. That is, the channels are monitored in parallel. One application of the multi-channel wavelength monitor of the present invention is high-speed parallel wavelength monitoring of many ITU channels in a DWM optical communication system. Another advantage of the multi-channel wavelength monitor of the present invention is that it uses a single optical filter to discriminate wavelength bands or channels and, therefore, is more compact and simpler to manufacture.
Accordingly, the present invention features a multi-channel wavelength monitor that, in one embodiment, substantially simultaneously monitors a plurality of optical channels. The monitor includes a dispersive element that is positioned in an optical path of an incident optical beam having a plurality of wavelengths. The dispersive element disperses the optical beam into a plurality of optical beams that simultaneously propagate in a plurality of optical paths, where each of the plurality of optical beams has one of the plurality of wavelengths. In one embodiment, the dispersive element includes an optical beam-shaping element.
An optical filter is positioned to intercept each of the plurality of optical paths at a plurality of locations. In one embodiment, a substantially transparent substrate is positioned between the dispersive element and the filter. At least one of an incident surface and an exit surface of the substrate may be anti-reflection coated. In one embodiment, the dispersive element and the optical filter are one optical element.
The optical filter substantially passes a respective one of the plurality of optical beams at a respective one of the plurality of locations and substantially blocks the other optical beams. The optical filter may be a thin film filter, such as a multi-cavity thin film filter. In one embodiment, the optical filter exhibits a spectral response that is dependent upon an incident angle at which each of the optical paths enter the optical filter. In another embodiment, the optical filter comprises a plurality of optical filters.
A plurality of optical detectors is positioned adjacent to the optical filter in a direction of propagation of the plurality of optical beams. A respective one of the plurality of optical detectors is positioned in a respective one of the plurality of optical paths. A respective one of the plurality of detectors generates an electrical signal that is proportional to an intensity of a respective one of the plurality of optical beams. In one embodiment, the detector comprises a photodiode array. The photodiode array may be periodically spaced or may be non-periodically spaced.
The present invention also features another multi-channel wavelength monitor that, in one embodiment, substantially simultaneously monitors a plurality of optical channels. The monitor includes a dispersive element that is positioned in an optical path of an incident optical beam having a plurality of wavelengths. The dispersive element disperses the optical beam into a plurality of optical beams that simultaneously propagates in a plurality of optical paths, where each of the plurality of optical beams has one of the plurality of wavelengths. In one embodiment, the dispersive element includes an optical beam-shaping element.
The monitor includes a plurality of optical filters where a respective one of the plurality of optical filters is positioned to intercept a respective one of the plurality of optical paths. A respective one of the plurality of optical filters substantially passes a respective one of the plurality of optical beams and substantially blocks each of the other optical beams. In one embodiment, the optical filters are thin film optical filters.
A plurality of optical detectors is positioned adjacent to the optical filter in a direction of propagation of the plurality of optical beams. A respective one of the plurality of optical detectors is positioned in a respective one of the plurality of optical paths. Each of the plurality of detectors generates an electrical signal that is proportional to an intensity of a respective one of the plurality of optical beams. In one embodiment, the detector comprises a photodiode array. The photodiode array may be periodically spaced or may be non-periodically spaced.
The present invention also features a method for simultaneously monitoring multiple wavelengths in a multi-channel optical beam. The method includes dispersing an optical beam into a plurality of optical beams that propagate in a plurality of optical paths, where each of the plurality of optical beams has one of a plurality of wavelengths. The plurality of optical beams is then transmitted through an optical filter. A respective one of a plurality of locations in the optical filter substantially transmits a respective one of the optical beams having a respective one of the plurality of wavelengths and substantially rejects other optical beams.
Each of the plurality of transmitted optical beams is simultaneously detected and a plurality of electrical signals is generated. A respective one of the plurality of electrical signals corresponds to an intensity of a respective one of the plurality of optical beams. The electrical signals are used to characterize the channels of the multi-channel optical beam. For example, the electrical signals can be used to determine the power in each channel of the optical beam. The transmitted optical beams may be detected by differentially detection.